Generally, ceramics have excellent heat resistance, corrosion resistance, chemical resistance, hardness, abrasion resistance, and insulating property, and intensive research has been carried out into the use of ceramics as structural parts by utilizing these excellent characteristics.
Silicon nitride (Si.sub.3 N.sub.4) is a material especially suitable for a part for which a high strength and a high abrasion resistance at high temperature are required, such as a gas turbine rotor or a fan blade liner or for a corrosion-resistant lining member of a chemical reaction tube to be in contact with a molten metal or the like.
But, in the case of ceramics, the integral production of a part having a complicated shape or mechanical processing is still difficult, and since ceramics are inherently brittle materials, the reliability of the strength thereof is inferior to that of metals. Accordingly, the production of products having a complicated shape by bonding ceramic materials or the development of composite structures having both the strength of a metal and the above-mentioned characteristics of a ceramic material by bonding the ceramic material to the metal is desired.
As the method for bonding ceramic materials to each other or bonding a ceramic material to a metal, there are adopted at present a mechanical bonding method such as screwing, shrink fitting or so-called fitting, an adhesive method using an organic or inorganic adhesive, a solder method in which the surface of a ceramic material is metallized and bonding is effected by using a solder, and a solid phase diffusion method in which bonding is effected at a high temperature under direct compression or through an insert. When a ceramic material is bonded to a metal according to the solid phase diffusion method, a flexural bonding strength of at least 100 MPa and occasionally 500 MPa (which is equal to the strength of the ceramic material per se) is obtained, and the residual stress generated by the difference of the thermal expansion coefficient is moderated by an insert interposed between the ceramic material and metal. Accordingly, the characteristics of the ceramic material and metal can be exerted most effectively in the solid phase diffusion method. Nevertheless, in the solid phase diffusion method, since a hot isostatic pressing (HIP) or a hot pressing (HP) must be used, the cost is increased. According to the solder bonding method, a metal layer is formed between a ceramic material and an adherend and the assembly is heated to fuse the metal layer and effect bonding. The strength and air tightness of the bonded portion are excellent and the bonding operation can be easily accomplished. In the case where a ceramic material is bonded to a metal or ceramic materials are bonded together according to this method, the surface of the ceramic material must be metallized, and as the wetting property of this metallized layer with the ceramic material is high, the bonding work quantity of the interface is increased and a strong bonding layer can be obtained.
But, since silicon nitride has a stron covalent bonding property, the affinity with a melt, that is, the wettability, is low. Furthermore, silicon nitride has a low reactivity with various substances. Therefore, a good metallized layer cannot be formed, and it is difficult to bond a ceramic material to a metal or ceramic materials to each other easily and firmly.
In view of the foregoing defects of the conventional techniques, it is a primary object of the present invention to provide a bonding composition capable of bonding ceramic materials such as silicon nitride (Si.sub.3 N.sub.4) to each other or bonding a ceramic material to a metal easily and firmly, and a bonding method using this bonding composition.